The Guardians Cats of the Sailor Scouts, Luna and Artemis, were loyal servants in Queen Serenity's Moon Kingdom during the Silver Millennium. When the Queen sent her daughter and other members of the royal court one thousand years into the future to be reborn, she also placed Luna and Artemis in a state of suspended animation to accompany them. When the cats awoke in the 20th century in separate locations, they did not fully retain memories of their lives in the Moon Kingdom. They did remember, however, that their purpose for existing was to summon together the Sailor Scouts and locate the lost Moon Princess.

In 1991, Artemis located the first Champion of Justice, Mina Aino, and gave her a Transformation Pen to help fight crime as Sailor V (she became Sailor Venus at a later date). In 1992, Luna located Sailor Moon, Sailor Mercury, Sailor Mars, and Sailor Jupiter, and told them of their destiny to battle the villains of the Negaverse. Luna and Artemis were in communication with each other during that time through a computer, although Luna did not know that the "Central Control" to whom she had been relaying information was actually Artemis. In October of 1992, Artemis brought Sailor Venus to meet Luna and the other Scouts so they could all work together to find the Moon Princess. Only days later, the Imperium Silver Crystal reformed and revealed that the Princess was actually Sailor Moon.

Luna and Artemis assist the Sailor Scouts in several ways: they provide the Scouts with their Transformation Pens and pocket/wrist communicators (updating them occasionally), they offer sound advice and encouragement, they teach the Scouts about their enemies and their Senshi powers, they use their resources to assist the Scouts in their missions, and they even enter combat occasionally if the Scouts are in great danger.

In 30th century Crystal Tokyo, Luna an Artemis become parents for the first time. Their kitten, Diana, serves as the Guardian Cat for Chibi-Usa (Rini) in Crystal Tokyo, and when Chibi-Usa returns to the 20th century (fourth season, Sailor Moon SuperS).